This invention relates to a control system for a direct-coupling mechanism installed in a hydraulic power transmission means of an automotive transmission, and more particularly to a control system of this kind which is intended to curtail the fuel consumption, as well as reduce vibrations of the vehicle body and operating noise of the engine over the whole operating region of the vehicle from a low vehicle speed region to a high one.
In an automotive vehicle equipped with a hydraulic power transmission means as a transmission, such as a hydraulic torque converter and a hydraulic coupling (hereinafter merely called "the torque converter") in general, the torque converter can provide due to its torque amplifying function a required driving force and a smooth and easy driving feeling over the whole speed region of the vehicle, even with a small number of speed reduction gears provided in the transmission. However, slippage loss inherent in the torque converter can cause degradation in the effective consumption of fuel and an increase in the engine rotational speed by an amount corresponding to the slippage loss, the latter resulting in larger operating noise of the engine.
To avoid this disadvantage, a direct-coupling or locking-up mechanism has been developed and actually brought into use which is adapted to mechanically couple the input and output members of the torque converter together to allow mechanical transmission of all or part of the engine power to the vehicle when the hydraulic power transmission by the torque converter is not necessary, thereby reducing the slippage loss for improvement of the power transmission efficiency.
In order to make best use of the direct-coupling mechanism to improve the effective fuel consumption and reduce the operating noise of the engine, it has been desired to expand the vehicle speed region wherein the direct-coupling mechanism is operated (hereinafter called "the direct-coupling operating region") to a lower speed side.
Conventionally, it is a general practice to render the direct-coupling mechanism inoperative to interrupt the locking-up of the torque converter and resume the hydraulic power transmission by the torque converter when the accelerator pedal is suddenly returned toward its idle position, in order to avoid torque fluctuations and accordingly vibrations of the whole transmission system, which would otherwise be caused by suddenly releasing the accelerator pedal to give discomfort to the driver and passenger(s). To effectively avoid such unpleasant vibrations due to torque fluctuations, it is desirable to disengage the direct-coupling mechanism immediately after the accelerator pedal is released when the throttle valve opening has not yet decreased to a much smaller value. However, conventionally such disengagement of the direct-coupling mechanism is generally effected when the throttle valve opening is reduced below a certain value, regardless of the vehicle speed.
A problem is encountered when the direct-coupling operating region is expanded to a low/middle vehicle speed region for the aforementioned purposes. That is, the throttle valve opening required at cruising in such low/middle vehicle speed region wherein the direct-coupling mechanism should be operative is much smaller than at cruising in a high vehicle speed region, and accordingly the throttle valve opening at which the direct-coupling mechanism is to be disengaged has to be set at a very small value below the above valve opening at cruising in the low/middle vehicle speed region. If such very small valve opening value is applied as the direct-coupling interrupting value throughout a whole operating region from a low vehicle speed region to a high one as conventionally adopted, there can be a delay in the disengagement of the direct-coupling mechanism when the accelerator pedal is suddenly released during running of the vehicle at a high speed, causing unpleasant vehicle body vibrations.